1. Field of the Invention
The present invention relates to a surface mounted device (SMD) type package using coaxial cables and, more particularly, to an SMD type package using more than one coaxial cable for conveniently transferring high frequency signals and for being easily attached to a printed circuit board (PCB).
2. Description of the Related Art
A conventional optical module includes an optical device and a package unit that is electrically connected to the optical device and is used for protecting the optical device. The optical module as a necessary component in an optical communication system, operates as an electric signal to an optical signal (E/O) and to an O/E converter. In detail, the optical module converts the electric signal of a base station, which is an input into master equipment of an optical repeating system, into an optical signal. Subsequently, the optical signal is transferred to slave equipment installed in an area where service is provided through optical cables. There is an increasing demand for optical modules capable of transmitting increasingly larger amounts of data at increasing higher speed. However, the amount of data and speed of its transfer are limited by structural problems of the package which protects the optical device and transfers high frequency signals.
In order for the package of the module to be suitable for the transmission of high frequency signals, it must meet the following conditions: first, the structure has to be simple so that it's members can be manually arranged and positioned at predetermined positions respectively; second, the package has to be conveniently bonded to a PCB to transfer the signals efficiently; third, the package must have a sufficient thermal capacity for minimizing thermal shock to the electronic device caused by internal and external heat of the module; and fourth, automation has to be achieved and a packaging cost, which is the most expensive part of manufacturing the module, has to be low.
A butterfly-type package is commonly used in conventional optical communication. However, the butterfly-type package is limited in its ability to transfer high frequency signals, so various types of alternative packages have been suggested. An example of an alternative package is shown in FIG. 1. In this package, via holes 13 are formed on a package unit 12 for connecting signal transfer lines. Namely, the package unit 12, which is formed of the via holes 13 used for transferring signals and of a ball grid array (BGA) 14 used for bonding the via holes 13 to a PCB, is mounted on an optical device. In this case, the optical device includes a silicon optical bench 11 on which optical components are mounted and a laser diode driver for controlling the optical components. Signals are transferred through the via holes 13, and the BGA bonding is performed under the via holes 13.
The package shown in FIG. 1 has several strengths. The size of the package can be reduced, and the package has a small amount of parasitic capacitance due to very small contact points. Moreover, unlike the butterfly-type package, the bonding is conveniently performed by using the via holes regardless of the number of contact points.
However, when the package using the via holes 13 as shown in FIG. 1 transfers high frequency signals, transfer loss occurs. For that reason, the via holes should be formed to have a smaller diameter. Moreover, the package has to have a sufficiently high thermal capacity for protecting optical devices that are sensitive to temperature. If the thermal capacity of the package is too low, the device inside may fall into a thermal shock.
As described above, the thermal capacity of the package is reduced when the via holes are formed to have a smaller diameter; thereby, the package insufficiently protects the optical device inside. Moreover, in the case of using the via holes, direct current (DC) signals are smoothly transferred, but it is difficult to transfer high frequency signals of higher than 40 GHz, which recently have been demanded by the industry.